In the majority of chemical processes there is a heat demand or a need to dissipate heat. Therefore, a wide range of chemical plant is involved in containing or conveying fluids which must at some stage of the process be either heated or cooled. One might consider furnaces, evaporators, distillation units, dryers and reaction vessels as plant where heat transfer manifests itself as a design and operational problem. In particular many industrial chemical processes employ reactors in which reactions are effected under given temperature and pressure conditions in the presence of a catalyst. Almost all these reactions generate or absorb heat i.e. they are exothermic or endothermic. The cooling effects for endothermic reactions generally adversely affect the rate of reaction and the corresponding parameters such as conversion and selectivity of the products from the reaction. The uncontrolled heating of exothermic reactions generally leads to damage to the associated apparatus as the temperature can rise to a very high level. The reaction in such a case may become uncontrolled (so-called “run away reaction”) and lead to unwanted by-products and undesired effects, such as deactivation of a process catalyst. Furthermore, whilst an ideal catalyst does not theoretically participate in a reaction in reality many catalysts become degraded or poisoned as the reaction progresses and on an industrial scale the costs associated with catalyst regeneration or replacement represent a significant burden. It will be understood that such costs also must include the down time for the plant or restrictions on capacity if a particular reactor has to be off-line for catalyst re-generation purposes. Therefore, it is desirable to prolong the life of a catalyst bed in view of the significant cost benefits that may be obtained overall. The invention to be described hereinafter is ideally suited to use in catalytic reactor design but can be adapted for other purposes. No distinction is made as to the application thereof to batch or continuous reaction systems.
Those skilled in the art recognise that it is beneficial for the changes in temperature resulting from the heating or cooling effects of the reactions to be controlled. It is well known that maintaining the temperature of the reaction at a given constant level may result in significant advantages to the reaction, such as improved conversion and selectivity, prolonged life of the catalyst and associated apparatus, reduced levels of unwanted by-products etc. In some cases, a varying of the constant temperature profile may be more beneficial.
In order to effectively control the temperature of reactions within an acceptable range, the chemical industry has devised several arrangements, those commonly used being discussed in standard references and texts e.g. one might consider the general teachings by Octave LEVENSPIEL in Chapter 19 of Chemical Reaction Engineering. The relative merits of each approach is also discussed therein.
Conventionally, the temperature inside reactors has been controlled by passing an auxiliary heat exchanging fluid through tubes or between plates, same forming a heat transfer conductive medium or thermal bridge whilst separating the reaction species from the auxiliary heat exchange fluid. Thus it will be understood that in such an indirect heat transfer system there is on the one hand a process path or zone and on the other an auxiliary fluid path or zone separated by the tube wall(s) or plate surface(s).
Considering this well known concept in relation to packed catalytic bed reactors, reactant fluid is passed through the catalyst bed and heat of reaction therein is controlled by contacting the catalytic bed reaction zone with such auxiliary fluid containing tubes or plates. However, particularly for highly exothermic reactions, such an approach has not been found to be ideal since the packed bed often develops heat gradients, e.g. the catalyst bed will be cooler at its areas of contact with the said tubes or plates and hotter within its depth remote from said tubes or plates, permitting formation of hot spots or moving hot fronts leading to variations in reaction progress within the bed as a whole. Thus at such a hot spot the reaction may proceed faster and hence catalyst therein will be more rapidly degraded. This will be particularly significant in the case of large plants.
Thus, it may be considered that the problems to be solved include the need to keep reactants and catalyst within a satisfactory temperature range with a view to maximising reaction rate; minimising reactor or catalyst volume; maximising yield of desired products, minimising damage to catalyst (e.g. due to excessive heat, or direct contamination with liquid phases); and minimise by-product formation.
Typical approaches to such problems include the addition of quench gas to cool the system, but this leads to a loss in efficiency and may thus have an adverse effect on yield. A further approach is to introduce a heat exchange step between adiabatic beds, which may involve the incorporation of heat exchangers into the reactor, but this leads to both design and operational problems due to the bulk of tubular designs and a lack of differential pressure containment with plate exchangers. There are also problems with reactant re-distribution. Alternatively, the reactants may be removed from the reactor for intermediate heat exchange e.g. cooling, but this also has an impact on plant design and process operation due to the additional expensive piping, and distribution problems with each extraction and re-injection. Thus this is not practical to do more than once or twice in any particular system.
A further approach is to adopt continuous heat exchange by packing tubes or plates into the catalyst bed, but this leads to design inflexibility, additional expense, uneven packing of catalyst, and of course difficulty in replacing or regenerating the catalyst. Alternatively, heat exchange tubes, plates and passages of the reactor may be coated with catalyst, but here again this leads to an inflexible design, difficulties in applying catalyst reliably and restrictions upon available superficial catalyst surface area. Also with this approach, there are obvious difficulties in replacing or regenerating the catalyst.
Fluidised bed reactors represent another potential solution to these problems, but these may not be ideally suited to all or certain reaction systems.
The possibility of using an inert or reactive diluent to ballast the temperature of reactants in adiabatic beds has been considered but such a diluent must be heated, cooled, and pumped which places extra energy demands on the process and moreover, may also interfere with the intended reaction by presenting a diffusion barrier to reactants.
The problems may be further explained by considering the principles of the staged adiabatic packed bed reactor system which is an example of an arrangement designed to offer more control over the reactant temperature. This system uses an arrangement wherein a number of discrete, spaced apart zones of reaction are provided with means therebetween to control the temperature of the products leaving a first zone of reaction prior to entering the next reaction zone. No heat exchanging means is provided to control the temperature of the reaction in the zones of the reaction. Thus the reactant fluid entering the reactor at a desired temperature passes through a packed bed containing catalyst. Upon exiting this first stage, the reactant gas and any products will have a temperature higher or lower than that of the initial temperature depending upon the reaction thermal characteristics. A heat exchanger then heats or cools the reactant gas to a second desired temperature, which may or may not be equivalent to the temperature of the first, before passing to the next packed bed i.e. the second stage. This sequence is repeated until the desired conversion is obtained. Thus the temperature profile of the reaction will be stepped within an acceptable range of temperature, and will therefore not be truly isothermal.
An alternative proposal for a process and apparatus for controlling reaction temperatures is disclosed in U.S. Pat. No. 5,600,053. This arrangement uses corrugated heat exchange plates spaced apart with each plate defining a boundary of a heat exchange flow channel on one side of the plate and a boundary of a reaction flow channel on the other. In the arrangement, a heat exchange fluid passes in the first of the aforementioned channels and a reactant stream passes through the second, preferably with a catalyst being present. This arrangement is intended to eliminate or minimise the typical step-wise approach to the so-called isothermal condition objective.
However, the arrangement proposed in U.S. Pat. No. 5,600,053 requires adjacent corrugated plates to be joined together. For this purpose, smooth edges are provided to facilitate the assembly of superposed multiple plates to form channels. The plates are joined, such as by welding, along these smooth edges and hence the integrity of the seal of the channels formed by the corrugations in adjacent plates is not ideal, particularly where a large pressure differential exists between the heat exchange flow channels and the reaction flow channels since this will tend to urge the adjacent plates apart. This arrangement will thus place unnecessary constraints on parameters of the reaction, namely the relationship between the pressure of the heat exchanging fluid and that of the reactant gas.
An earlier system is described in U.S. Pat. No. 5,073,352 which proposes an apparatus for conducting a process of reforming gasolines, under low pressure and in the presence of at least one catalyst, in which heat required for the reaction is provided by a heat carrying fluid such as natural gas.
The apparatus described therein comprises a number of discrete reaction cells being arranged vertically and being of substantially parallelepipedic configuration. The cells are laterally spaced apart, thus forming channels therebetween for flow of the heat carrying fluid. The reforming catalyst-containing chambers are respectively either isothermal or adiabatic and dimensioned such that height (H), width (W) and thickness (T) satisfy the conditions H>W>T, and H is at least twice the value of W, W lying in the range of 50 mm to 10,000 mm (0.05–10 meters) and T lies in the range of 2 mm 2,000 mm (0.002–2 meters). Thus there remains the possibility of hot spots and less than satisfactory thermal control in such large catalytic reactor volumes.
It is known to the man skilled in the art that the heat transfer coefficient in a packed bed is mainly dependent upon the catalyst particle size and the reactant fluid velocity through the catalytic bed. Unfortunately, both these parameters are process requirements and hence cannot be changed in order to improve the heat transfer coefficient in the packed bed, and hence in the reaction cells described in U.S. Pat. No. 5,073,352. Additionally, it is difficult to move catalyst between narrow gaps or tubes, imposing limits on the dimensions of gaps or tubes through which catalyst is designed to flow.
Thus reactors of the known types according to the existing art have many significant limitations imposed on the heat transfer capability. Plate reactors offer some advantages over tubular reactors on the auxiliary medium side, but the end result is not significant since the overall heat transfer coefficient is generally governed by the process side as discussed above. Tubular reactors on the other hand offer advantages over plate reactors with regard to mechanical capability, due to increased resistance to differential pressure between the reactant fluid and the heat exchanging fluid.
There are also industrial chemical processes where the reaction is performed without the presence of a catalyst. Such reactions are frequently carried out in the liquid phase in a stirred tank (a so-called CSTR reactor), and may be continuous or batch or semi-batch processes.
Heating or cooling of CSTR reactors is typically either by an external heat transfer jacket, an internal heat transfer coil, or by circulating the reactants through an external heat exchanger. Generally both an external jacket and internal coil afford only a limited heat transfer surface. External heat transfer requires pumped circulation, and imperfect mixing within the reactor can result in significant deviation between the composition of the fluid circulated through the external heat exchanger and bulk mixed fluid composition in the reactor. This latter problem is most likely to arise whilst a reactant is being added to the reactor.
Fluid mixing within a CSTR is dependent on such factors as the agitator and baffle design, agitator speed, and the physical properties of the reactant mixture. Efforts to ensure good mixing frequently meet with unsatisfactory results, and reaction conditions often need to be less than optimal to allow for variations in reactant concentration.
As a result of the shortcomings outlined above, some reactions are carried out with a reactor residence time that is much longer than needed by the reaction kinetics alone, due to poor heat and/or mass transfer, with consequent loss of reaction selectivity. Further, CSTRs are characterised by substantially complete back mixing of reaction products with reactant; this can inhibit the reaction rate, and can also result in product loss through unwanted further reactions. The invention enables a fast, heat transfer-limited reaction to be carried out with a much reduced residence time, in a sequential series of low residence time stages which approximates to a so-called plug flow reactor.
Considering the foregoing matters, it is an object of this invention to provide improvements in chemical plant design and methods of operation thereof with a view to obviating or mitigating the drawbacks of the existing or previously proposed designs and methods.
Particularly, it is an aim of the present invention to provide an apparatus and a process for the control of reaction temperature within an acceptable range during operation of the chemical process by an indirect heat transfer method using a heat exchanging fluid.
Another object of the invention to be described more particularly hereinbelow is to provide an apparatus permitting control of the reactant temperature closely within a desired profile, more specifically, aiming to maintain the temperature at a substantially constant level i.e. to offer attainment of an acceptable approximation to an isothermal reaction zone in so far as is practical on an industrial scale.
It is a further object of the invention to provide chemical plant which is improved over known plant equipment in terms of both cost and space efficiency considerations.